Roar of triumph is a feature granted to member of the Barbarian class that choose the Thaneborn Triumph Feral Might option.
Roar of Triumph
Barbarian feature
Trigger: your attack reduces an enemy to 0 hit points
Target: each enemy in burst
Effect: "Each target takes a −2 penalty to all defenses until the end of your next turn." [PH2:50]
Your howl of victory shakes your enemies to the core, as they know your blood thirst is not yet quenched.